Annular cutters of the type which include a barrel in a cylindrical form and cutting teeth attached at regular intervals to the lower portion of a housing are disclosed in Japanese Utility Model Kokai Publication Nos. 57-100415 (1982) and 57-100416 (1982). The cutting teeth are partly formed of a hard material. A groove is defined between the adjacent teeth for the removal of chips. The annular cutter is designed to form a hole in the workpiece by allowing a cylindrical slug to drop through. Such a type of annular hole cutter can cut a hole larger than that formed by a twist cutter, since the hole-cutting or drilling is effected while drilling an annular hole having a width corresponding to the thickness of the hard teeth. Furthermore, since the cutting area of the cutting teeth is reduced by dividing the cutting teeth into several groups, smaller resistance to cutting and faster cutting speed are achieved.
A problem is involved in the use of the above described annular cutter. More specifically, the deeper the depth of the hole being cut, the more difficult the removal of chips. For that reason, minute chips tend to be jammed or wedged in between the inner portions of the cutting teeth and a cylindrical core formed during drilling. The core drops off as a slug after the completion of the drilling. A representative slug 14 is illustrated in FIG. 4. When jamming of chips becomes significant, there is an increase in the resistance to cutting, which may eventually lead to break-down of the annular cutter. Alternatively, there is an increase in the load applied to an electric motor for driving the cutter, which may ultimately result in burning-out of the motor. In some cases, the heat of friction generated by an increase in the resistance to cutting may give rise to a melting of the chips. The chips are in turn deposited and cured onto the inner cutting teeth. As a consequence, the cutting edges will chip or break down, or burning-out of the motor will take place.